In general, an X-ray CT apparatus in the related art has a configuration in which an X-ray detector in a current mode, in which energy information is not obtained, detects an X-ray photon group having a continuous (non-monochromatic) energy distribution that is generated by an X-ray tube. An X-ray attenuation coefficient has energy dependency that is specific to each of elements having different atomic numbers, and thus different energy distributions make it possible to obtain information related to the atomic numbers from a plurality of attenuation coefficients. However, in the general configuration of the X-ray CT apparatus in the related art, it is not possible to acquire information related to the atomic numbers.
In recent years, a technology for effectively using information by X-ray groups having a plurality of energy distributions has been developed, and examples of the technology include the following two technologies, largely. One technology is referred to as dual energy CT in which an X-ray detector has the current mode as is and uses two types of continuous energy distributions of X-rays, which are generated by two types of X-ray tube voltages. In addition, the other technology is referred to as photon counting CT (PCCT), spectral CT or the like and uses a pulse mode detector that is capable of acquiring energy information.
The X-ray CT apparatus finds a difference in ability of a substance for blocking an X-ray (an attenuation coefficient); however, dependency of an X-ray attenuation coefficient on energy varies depending on an element (atomic number). Hence, in a case of obtaining N types of energy information, it is possible to perform substance resolution into optional M (M≤N) substances having different effective atomic numbers, as base substances (for example, see PTL 1). Depending on whether the substance resolution is performed before returning to an image from projection information or after returning to the image, an image reconstruction method thereof includes two methods of a projection base method (pre-reconstruction method) and an image base method (post-reconstruction method) (for example, see NPTL 1). In the PCCT under a high-flux condition, a detection element is finer than that in the related art, and thus the projection base method is applied in order to process a complex spectral response, in general.